Post RIP image rendering in an electrographic printer to estimate toner consumption

ABSTRACT

A method of estimating toner consumption of a input digital image when printed, the digital image comprised of an array of pixels and wherein each pixel is assigned a digital value representing marking information, the method comprising the steps of identifying each pixel as background pixels or foreground pixels; adding the digital values of foreground pixels together; and, estimating toner usage based on the sum of the added values. The rendering of the present invention occurs post RIP.

RELATED APPLICATIONS

This application claims the priority date of U.S. ProvisionalApplication Ser. No. 60/459.114 filed Mar. 31, 2003 entitled “POST RIPIMAGE RENDERING IN AN ELECTROGRAPHIC PRINTER TO ESTIMATE TONERCONSUMPTION”.

FIELD OF THE INVENTION

This invention is in the field of digital printing, and is morespecifically directed to image exposure control in electrostatographicprinters.

BACKGROUND OF THE INVENTION

Electrographic printing has become the prevalent technology for modemcomputer-driven printing of text and images, on a wide variety of hardcopy media. This technology is also referred to as electrographicmarking, electrostatographic printing or marking, andelectrophotographic printing or marking. Conventional electrographicprinters are well suited for high resolution and high speed printing,with resolutions of 600 dpi (dots per inch) and higher becomingavailable even at modest prices. As will be described below, at theseresolutions, modem electrographic printers and copiers are well-suitedto be digitally controlled and driven, and are thus highly compatiblewith computer graphics and imaging. Controlling the appearance ofprinted images is an important aspect of printers. An example of suchcontrol efforts is described in U.S. Pat. No. 6,181,438, which is herebyincorporated herein by reference.

Efforts regarding printers or printing systems have led to continuingdevelopments to improve their versatility practicality, and efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a–1 b are schematic diagrams of an electrographic marking orreproduction system in accordance with the present invention.

FIG. 2 is a schematic block diagram for image rendering in accordancewith the present invention.

FIG. 3 is a flow chart for image rendering in accordance with thepresent invention.

FIG. 4 is a schematic diagram of designations for a directional look uptable in accordance with the present invention.

FIGS. 5 a–5 d are representations of a pixel grid having a toned imageprovided thereon in accordance with the present invention.

FIGS. 6 a–6 f are representations of a pixel grid having an imageprovided thereon in accordance with the present invention.

FIG. 7 a is a representation of a pixel grid having a one pixel widetoned image provided thereon in accordance with the present invention.

FIG. 7 b is a representation of a pixel grid with edge pixeldesignations for the toned image of FIG. 7 a in accordance with thepresent invention.

FIG. 7 c is a representation of a pixel grid with direction values forthe toned image of FIG. 7 a in accordance with the present invention.

FIG. 7 d is a representation of a pixel grid with background pixel, edgepixel and one pixel wide line assignment values for the toned image ofFIG. 7 a in accordance with the present invention.

FIG. 8 a is a representation of a pixel grid having a two pixel widetoned image provided thereon in accordance with the present invention.

FIG. 8 b is a representation of a pixel grid with edge pixel assignmentsfor the toned image of FIG. 8 a in accordance with the presentinvention.

FIG. 8 c is a representation of a pixel grid with direction values forthe toned image of FIG. 8 a in accordance with the present invention.

FIG. 8 d is a representation of a pixel grid with background pixel, edgepixel and two pixel wide line assignment values for the toned image ofFIG. 8 a in accordance with the present invention.

FIG. 9 is a schematic representation of an exemplary adjustmentinterface for assigning new pixel values according to the presentinvention.

FIG. 10 is a representation of a pixel grid with alternative pixelassignments in accordance with the present invention for a letter O.

FIG. 11 is an example of a tone reproduction curve for an electrographicprinter in accordance with the present invention.

FIG. 12 is a copy of a series of printed halftone steps for threedifferent screen frequencies.

FIG. 13 is a graph illustrating percent lightness vs percent blackpixels for each step for each screen frequency shown in FIG. 14.

FIG. 14 is a copy of a series of printed lines that are 1, 2, 3, 4, and8 pixels wide.

FIG. 15 is a graph illustrating linewidth vs the number of pixelscounted across the line for an exemplary series of lines of FIG. 14.

FIG. 16 is a graph illustrating best fit lines extracted for linewidthvs the number of pixels derived by selecting a fixed IPV and varyingEPV, 2PV and 1PV for eight different cases.

FIG. 17 is a flow chart illustrating the steps taken to thin an objectby more than one pixel, in accordance with the present invention.

DETAILED DESCRIPTION

Referring to FIG. 1, a printer machine 10 includes a moving recordingmember such as a photoconductive belt 18 which is entrained about aplurality of rollers or other supports 21 a through 21 g, one or more ofwhich is driven by a motor to advance the belt. By way of example,roller 21 a is illustrated as being driven by motor 20. Motor 20preferably advances the belt at a high speed, such as 20 inches persecond or higher, in the direction indicated by arrow P, past a seriesof workstations of the printer machine 10. Alternatively, belt 18 may bewrapped and secured about only a single drum.

Printer machine 10 includes a controller or logic and control unit (LCU)24, preferably a digital computer or microprocessor operating accordingto a stored program for sequentially actuating the workstations withinprinter machine 10, effecting overall control of printer machine 10 andits various subsystems. LCU 24 also is programmed to provide closed-loopcontrol of printer machine 10 in response to signals from varioussensors and encoders. Aspects of process control are described in U.S.Pat. No. 6,121,986 incorporated herein by this reference.

A primary charging station 28 in printer machine 10 sensitizes belt 18by applying a uniform electrostatic corona charge, from high-voltagecharging wires at a predetermined primary voltage, to a surface 18 a ofbelt 18. The output of charging station 28 is regulated by aprogrammable voltage controller 30, which is in turn controlled by LCU24 to adjust this primary voltage, for example by controlling theelectrical potential of a grid and thus controlling movement of thecorona charge. Other forms of chargers, including brush or rollerchargers, may also be used.

An exposure station 34 in printer machine 10 projects light from awriter 34 a to belt 18. This light selectively dissipates theelectrostatic charge on photoconductive belt 18 to form a latentelectrostatic image of the document to be copied or printed. Writer 34 ais preferably constructed as an array of light emitting diodes (LEDs),or alternatively as another light source such as a laser or spatiallight modulator. Writer 34 a exposes individual picture elements(pixels) of belt 18 with light at a regulated intensity and exposure, inthe manner described below. The exposing light discharges selected pixellocations of the photoconductor, so that the pattern of localizedvoltages across the photoconductor corresponds to the image to beprinted. An image is a pattern of physical light which may includecharacters, words, text, and other features such as graphics, photos,etc. An image may be included in a set of one or more images, such as inimages of the pages of a document. An image may be divided intosegments, objects, or structures each of which is itself an image. Asegment, object or structure of an image may be of any size up to andincluding the whole image.

Image data to be printed is provided by an image data source 36, whichis a device that can provide digital data defining a version of theimage. Such types of devices are numerous and include computer ormicrocontroller, computer workstation, scanner, digital camera, etc.These data represent the location and intensity of each pixel that isexposed by the printer. Signals from data source 36, in combination withcontrol signals from LCU 24 are provided to a raster image processor(RIP) 37. The Digital images (including styled text) are converted bythe RIP 37 from their form in a page description language (PDL) languageto a sequence of serial instructions for the electrographic printer in aprocess commonly known as “ripping” and which provides a ripped image toa image storage and retrieval system known as a Marking Image Processor(MIP) 38.

In general, the major roles of the RIP 37 are to: receive jobinformation from the server; Parse the header from the print job anddetermine the printing and finishing requirements of the job; Analyzethe PDL (Page Description Language) to reflect any job or pagerequirements that were not stated in the header; Resolve any conflictsbetween the requirements of the job and the Marking Engine configuration(i.e., RIP time mismatch resolution); Keep accounting record and errorlogs and provide this information to any subsystem, upon request;Communicate image transfer requirements to the Marking Engine; Translatethe data from PDL (Page Description Language) to Raster for printing;and Support Diagnostics communication between User Applications The RIPaccepts a print job in the form of a Page Description Language (PDL)such as PostScript, PDF or PCL and converts it into Raster, a form thatthe marking engine can accept. The PDL file received at the RIPdescribes the layout of the document as it was created on the hostcomputer used by the customer. This conversion process is calledrasterization. The RIP makes the decision on how to process the documentbased on what PDL the document is described in. It reaches this decisionby looking at the first 2K of the document. A job manager sends the jobinformation to a MSS (Marking Subsystem Services) via Ethernet and therest of the document further into the RIP to get rasterized. Forclarification, the document header contains printer-specific informationsuch as whether to staple or duplex the job. Once the document has beenconverted to raster by one of the interpreters, the Raster data goes tothe MIP 38 via RTS (Raster Transfer Services); this transfers the dataover a IDB (Image Data Bus).

The MIP functionally replaces recirculating feeders on optical copiers.This means that images are not mechanically rescanned within jobs thatrequire rescanning, but rather, images are electronically retrieved fromthe MIP to replace the rescan process. The MIP accepts digital imageinput and stores it for a limited time so it can be retrieved andprinted to complete the job as needed. The MIP consists of memory forstoring digital image input received from the RIP. Once the images arein MIP memory, they can be repeatedly read from memory and output to theRender Circuit. The amount of memory required to store a given number ofimages can be reduced by compressing the images; therefore, the imagesare compressed prior to MIP memory storage, then decompressed whilebeing read from MIP memory.

The output of the MIP is provided to an image render circuit 39, whichalters the image and provides the altered image to the writer interface32 (otherwise known as a write head, print head, etc.) which appliesexposure parameters to the exposure medium, such as a photoconductor 18.

After exposure, the portion of exposure medium belt 18 bearing thelatent charge images travels to a development station 35. Developmentstation 35 includes a magnetic brush in juxtaposition to the belt 18.Magnetic brush development stations are well known in the art, and arepreferred in many applications; alternatively, other known types ofdevelopment stations or devices may be used. Plural development stations35 may be provided for developing images in plural colors, or fromtoners of different physical characteristics. Full process colorelectrographic printing is accomplished by utilizing this process foreach of four toner colors (e.g., black, cyan, magenta, yellow).

Upon the imaged portion of belt 18 reaching development station 35, LCU24 selectively activates development station 35 to apply toner to belt18 by moving backup roller 35 a belt 18, into engagement with or closeproximity to the magnetic brush. Alternatively, the magnetic brush maybe moved toward belt 18 to selectively engage belt 18. In either case,charged toner particles on the magnetic brush are selectively attractedto the latent image patterns present on belt 18, developing those imagepatterns. As the exposed photoconductor passes the developing station,toner is attracted to pixel locations of the photoconductor and as aresult, a pattern of toner corresponding to the image to be printedappears on the photoconductor. As known in the art, conductor portionsof development station 35, such as conductive applicator cylinders, arebiased to act as electrodes. The electrodes are connected to a variablesupply voltage, which is regulated by programmable controller 40 inresponse to LCU 24, by way of which the development process iscontrolled.

Development station 35 may contain a two component developer mix whichcomprises a dry mixture of toner and carrier particles. Typically thecarrier preferably comprises high coercivity (hard magnetic) ferriteparticles. As an example, the carrier particles have a volume-weighteddiameter of approximately 30μ. The dry toner particles are substantiallysmaller, on the order of 6μ to 15μ in volume-weighted diameter.Development station 35 may include an applicator having a rotatablemagnetic core within a shell, which also may be rotatably driven by amotor or other suitable driving means. Relative rotation of the core andshell moves the developer through a development zone in the presence ofan electrical field. In the course of development, the toner selectivelyelectrostatically adheres to photoconductive belt 18 to develop theelectrostatic images thereon and the carrier material remains atdevelopment station 35. As toner is depleted from the developmentstation due to the development of the electrostatic image, additionaltoner is periodically introduced by toner auger 42 into developmentstation 35 to be mixed with the carrier particles to maintain a uniformamount of development mixture. This development mixture is controlled inaccordance with various development control processes. Single componentdeveloper stations, as well as conventional liquid toner developmentstations, may also be used.

A transfer station 46 in printing machine 10 moves a receiver sheet Sinto engagement with photoconductive belt 18, in registration with adeveloped image to transfer the developed image to receiver sheet S.Receiver sheets S may be plain or coated paper, plastic, or anothermedium capable of being handled by printer machine 10. Typically,transfer station 46 includes a charging device for electrostaticallybiasing movement of the toner particles from belt 18 to receiver sheetS. In this example, the biasing device is roller 46 b, which engages theback of sheet S and which is connected to programmable voltagecontroller 46 a that operates in a constant current mode duringtransfer. Alternatively, an intermediate member may have the imagetransferred to it and the image may then be transferred to receiversheet S. After transfer of the toner image to receiver sheet S, sheet Sis detacked from belt 18 and transported to fuser station 49 where theimage is fixed onto sheet S, typically by the application of heat.Alternatively, the image may be fixed to sheet S at the time oftransfer.

A cleaning station 48, such as a brush, blade, or web is also locatedbehind transfer station 46, and removes residual toner from belt 18. Apre-clean charger (not shown) may be located before or at cleaningstation 48 to assist in this cleaning. After cleaning, this portion ofbelt 18 is then ready for recharging and re-exposure. Of course, otherportions of belt 18 are simultaneously located at the variousworkstations of printing machine 10, so that the printing process iscarried out in a substantially continuous manner.

LCU 24 provides overall control of the apparatus and its varioussubsystems as is well known. LCU 24 will typically include temporarydata storage memory, a central processing unit, timing and cycle controlunit, and stored program control. Data input and output is performedsequentially through or under program control. Input data can be appliedthrough input signal buffers to an input data processor, or through aninterrupt signal processor, and include input signals from variousswitches, sensors, and analog-to-digital converters internal to printingmachine 10, or received from sources external to printing machine 10,such from as a human user or a network control. The output data andcontrol signals from LCU 24 are applied directly or through storagelatches to suitable output drivers and in turn to the appropriatesubsystems within printing machine 10.

Process control strategies generally utilize various sensors to providereal-time closed-loop control of the electrostatographic process so thatprinting machine 10 generates “constant” image quality output, from theuser's perspective. Real-time process control is necessary inelectrographic printing, to account for changes in the environmentalambient of the photographic printer, and for changes in the operatingconditions of the printer that occur over time during operation(rest/run effects). An important environmental condition parameterrequiring process control is relative humidity, because changes inrelative humidity affect the charge-to-mass ratio Q/m of tonerparticles. The ratio Q/m directly determines the density of toner thatadheres to the photoconductor during development, and thus directlyaffects the density of the resulting image. System changes that canoccur over time include changes due to aging of the printhead (exposurestation), changes in the concentration of magnetic carrier particles inthe toner as the toner is depleted through use, changes in themechanical position of primary charger elements, aging of thephotoconductor, variability in the manufacture of electrical componentsand of the photoconductor, change in conditions as the printer warms upafter power-on, triboelectric charging of the toner, and other changesin electrographic process conditions. Because of these effects and thehigh resolution of modem electrographic printing, the process controltechniques have become quite complex.

Process control sensor may be a densitometer 76, which monitors testpatches that are exposed and developed in non-image areas ofphotoconductive belt 18 under the control of LCU 24. Densitometer 76 mayinclude a infrared or visible light LED, which either shines through thebelt or is reflected by the belt onto a photodiode in densitometer 76.These toned test patches are exposed to varying toner density levels,including full density and various intermediate densities, so that theactual density of toner in the patch can be compared with the desireddensity of toner as indicated by the various control voltages andsignals. These densitometer measurements are used to control primarycharging voltage V_(O), maximum exposure light intensity E_(O), anddevelopment station electrode bias V_(B). In addition, the processcontrol of a toner replenishment control signal value or a tonerconcentration setpoint value to maintain the charge-to-mass ratio Q/m ata level that avoids dusting or hollow character formation due to lowtoner charge, and also avoids breakdown and transfer mottle due to hightoner charge for improved accuracy in the process control of printingmachine 10. The toned test patches are formed in the interframe area ofbelt 18 so that the process control can be carried out in real timewithout reducing the printed output throughput. Another sensor usefulfor monitoring process parameters in printer machine 10 is electrometerprobe 50, mounted downstream of the corona charging station 28 relativeto direction P of the movement of belt 18. An example of an electrometeris described in U.S. Pat. No. 5,956,544 incorporated herein by thisreference.

Other approaches to electrographic printing process control may beutilized, such as those described in International Publication Number WO02/10860 A1, and International Publication Number WO 02/14957 A1, bothcommonly assigned herewith and incorporated herein by this reference.

Raster image processing begins with a page description generated by thecomputer application used to produce the desired image. The Raster ImageProcessor interprets this page description into a display list ofobjects. This display list contains a descriptor for each text andnon-text object to be printed; in the case of text, the descriptorspecifies each text character, its font, and its location on the page.For example, the contents of a word processing document with styled textis translated by the RIP into serial printer instructions that include,for the example of a binary black printer, a bit for each pixel locationindicating whether that pixel is to be black or white. Binary printmeans an image is converted to a digital array of pixels, each pixelhaving a value assigned to it, and wherein the digital value of everypixel is represented by only two possible numbers, either a one or azero. The digital image in such a case is known as a binary image.Multi-bit images, alternatively, are represented by a digital array ofpixels, wherein the pixels have assigned values of more than two numberpossibilities. The RIP renders the display list into a “contone”(continuous tone) byte map for the page to be printed. This contone bytemap represents each pixel location on the page to be printed by adensity level (typically eight bits, or one byte, for a byte maprendering) for each color to be printed. Black text is generallyrepresented by a full density value (255, for an eight bit rendering)for each pixel within the character. The byte map typically containsmore information than can be used by the printer. Finally, the RIPrasterizes the byte map into a bit map for use by the printer. Half-tonedensities are formed by the application of a halftone “screen” to thebyte map, especially in the case of image objects to be printed.Pre-press adjustments can include the selection of the particularhalftone screens to be applied, for example to adjust the contrast ofthe resulting image.

Electrographic printers with gray scale printheads are also known, asdescribed in International Publication Number WO 01/89194 A2,incorporated herein by this reference. As described in this publication,the rendering algorithm groups adjacent pixels into sets of adjacentcells, each cell corresponding to a halftone dot of the image to beprinted. The gray tones are printed by increasing the level of exposureof each pixel in the cell, by increasing the duration by way of which acorresponding LED in the printhead is kept on, and by “growing” theexposure into adjacent pixels within the cell.

Ripping is printer-specific, in that the writing characteristics of theprinter to be used is taken into account in producing the printer bitmap. For example, the resolution of the printer both in pixel size (dpi)and contrast resolution (bit depth at the contone byte map) willdetermine the contone byte map. As noted above, the contrast performanceof the printer can be used in pre-press to select the appropriatehalftone screen. RIP rendering therefore incorporates the attributes ofthe printer itself with the image data to be printed.

The printer specificity in the RIP output may cause problems if the RIPoutput is forwarded to a different electrographic printer. One suchproblem is that the printed image will turn out to be either darker orlighter than that which would be printed on the printer for which theoriginal RIP was performed. In some cases the original image data is notavailable for re-processing by another RIP in which tonal adjustmentsfor the new printer may be made.

FIG. 2 illustrates a schematic block diagram of the function of rendercircuit 39. For exemplary purposes only, it is assumed that binary imagedata is provided by the RIP on line 310 to converter circuit 312 which,in this example, converts the data from binary to multi-bit data, suchas eight bit data. For example, the pixel value may be converted from a1 or 0 value, to a value ranging from 0 to 255 and provided on a line314. For simplicity, it will be assumed that the pixel being treated orthe pixel in question (PIQ) values on line 314 is either 0 or 255. The 8bit PIQ value is provided to an edge determination circuit 316 whichapplies a standard 3×3 edge Laplacian kernel circuit to determine if thePIQ is an edge pixel. The results (A) of this edge determination isprovided on a line 317 to mapping circuit 318 and a pixel object widthdetermination circuit 320. In other terminology, circuit 316 flagswhether the PIQ is edge pixel or not. An edge is defined as a transitionbetween background and foreground. Edge pixels define the transitionbetween background and foreground pixels. Background pixels are definedas pixels having relatively,little or no printable or markinginformation within. Print or marking information is the digital valueassigned to the pixel which results in a certain amount of markingmaterial, such as ink or toner, to be deposited on a receiver, where theamount of material has a functional relationship to the digital value.For example, in the present embodiment, higher digital values may meanhigher amounts of toner being deposited, resulting in a visually darkerpixel. An inverse relationship could also be employed, however.Foreground pixels are defined as pixels having some printable or markinginformation within. Foreground pixels may be either interior pixels,edge pixels, one line pixels, or two line pixels. Interior pixels areforeground pixels that are not edge pixels, one line pixels, or two linepixels.

The output on line 314 from converter circuit 312 is also provided to a3×3 directional lookup table circuit 322. Circuit 322 assigns adesignation or flag to the PIQ depending on the pixels surrounding it ina 3×3 matrix. With reference to FIG. 4, it can be seen that there areeight different configurations for an edge PIQ in the center of a 3×3pixel matrix. The PIQ is designated by an “X”. Each matrix is assignedan arbitrary designation, and in this example the designations areletters indicating direction of the adjoining pixels from the PIQ. Otherletters or numerical designations may just as well been assigned. Theoutput (D) of directional lookup table circuit 322 is provided on a line323 to character or object pixel width determination circuit 320.

Pixel width determination circuit 320 determines if the edge PIQ is partof an object that is one or two pixels wide, and flags the data with atag (B) accordingly on a line 321.

Mapping circuit 318 is provided information from multiple sources andprovides an output on a line 340 to the writer interface. The inputs tomapping circuit are the edge detection pixel information A on line 317,object width information B on a line 321, and original image PIQ data Con line 314. In addition, assignment values for interior pixels, edgepixels, one pixel wide lines and two pixel wide lines are provided tomapping circuit 318 on lines 330, 332, 334 and 336. These assignmentvalues are new values that will be given to the PIQ, depending uponwhether the PIQ is part of a two pixel wide object (2PV), one if the PIQis part of a one pixel wide object (1PV), one if the PIQ is an edgepixel of an object more than two pixels wide (EPV), and another value ifthe PIQ is an interior (not background) pixel (IPV). Background pixels(white area) are not changed by this particular algorithm, althoughanother might do so to achieve a desired effect.

The types of assignment parameters and the number of assignment valuesmay be determined in an unlimited number of ways. For example, they maybe provided by a user in response to a particular effect the printoperator wishes to obtain by programming through a user interface,mechanical switches or other adjustments. The assignment values may alsobe determined automatically by the controller or LCU in response toprinter operational parameters, operator input or other input. Theassignment values and parameters may be combined to determine newassignment parameters. However they may be determined, new pixel tone orexposure values will be assigned to the PIQ post rip. One primary factorin new pixel tone value assignment is the location of the PIQ in theimage in relation to surrounding pixels. Although the input to therendering circuit is explained as a binary input, the input may also bea multi-bit input wherein new multi-bit PIQ exposure values will beassigned for the input PIQ exposure values. Also, it is to be noted thatthe digital input PIQ exposure values may be either binary or multi-bit,meaning the input digital image may be either binary or multi-bit.

Rendering circuit 39 is an in line interface, or serial interface inthat it is provided between the RIP and the writer interface. Imagerendering can therefore be accomplished independent of the printer orother printer components discussed hereinbefore, such as the RIP orwriter interface. It may be implemented with hardware (such as acomputer or processor board), software, or firmware as those terms areknown to those skilled in the art. The image rendering of the presentinvention can also be accomplished utilizing data from the other printercomponents, such as data typically utilized for process control. Inaddition, image rendering may be set or programmed by an operator orother external or remote source in order to achieve a particular effector effects in the printed output. Implementing a rendering circuit inhardware just prior to gray level writer allows for lower bandwidthrequirements right up to last stage before exposure. The writer may beany grey level exposure system.

Referring to FIG. 3, a flowchart of a method of image rendering bycircuit 39 is provided. In a first step 210, binary image data isreceived from the data source 36, preferably after it has been ripped bythe RIP 37. In a step 212, circuit 39 determines whether the pixel beingtreated or the pixel in question (PIQ) is an edge pixel. Edge pixels ofbinary images may be detected using any of a number of standardalgorithms known in the art (William K. Pratt, Digital Image Processing,Second Edition, John Wiley and Sons, 1991, Chapter 16). The edge can bethe white edge or black edge. The black edge is used for “thinning” orlightening and the “white edge” is used for thickening or darkening. Todetect black edges, the binary image is converted to 8 bits (e.g. 0→0and 1→255) and a standard 3×3 edge Laplacian kernel is applied.Preferred embodiment uses the following kernel:

0 −1 0 −1 4 −1 0 −1 0

The result of this operation is an image in which all image pixels are 0except for edge pixels which have a value of 255. To detect white edges,the binary image is converted to 8 bits and inverted (e.g. 0→255 and1→0). White edges of text and other features are detected when the imageis to be darkened or lines and halftones dots are to be made wider. Thewhite pixel edges are then replaced with a gray level to widen or extendthe exposed region. The amount of gray level added determines the degreeto which the image is darkened. The particular edge detection algorithmutilized can be combinations and refinements of standard algorithmsknown in the art. In a thinning case, changing the edge pixels of eachhalftone cell to gray lightens the printed pictorial image. In athickening case, adding gray to the white edge pixels around thehalftone cell darkens the image.

If the PIQ is determined not to be an edge pixel, then in a step 214,the determination is made whether the PIQ value is zero or somethingother than zero. If the PIQ value is zero, then the PIQ value isassigned the background pixel value BPV, (which for exemplary purposesin this case is zero) in a step 215, since it is part of the background.If the PIQ value is not zero then it's assumed it's an interior pixel(solid area pixel) and a new interior pixel value (IPV) is assigned toit in a step 216.

If the PIQ in step 212 was determined to be an edge pixel, a step 217determines whether the image rendering is in a thinning mode or awidening mode. These modes will be discussed in more detail hereinafter.If a thinning mode is desired, then a determination is made in a step218 as to whether the PIQ is an edge pixel of a line or object that isone pixel wide. If yes, then the PIQ is assigned a new one pixel widevalue (1PV) in a step 220. If the answer to step 218 is no, then adetermination is made in a step 222 whether the PIQ is part of a line orobject that is two pixels wide. If yes, then a two pixel wide value(2PV) is assigned to the PIQ in a step 224.

If the answer to step 217 is no, then the PIQ is assigned an edge pixelvalue (EPV) in a step 226.

It is to be noted that the flowchart of FIG. 3 may be an algorithm thatis performed as part of the mapping circuit 318 or function asillustrated in FIG. 2. Also, as can be seen in FIG. 2, binary pixel datais provided by the RIP to the input of the image rendering circuit andmulti-bit pixel data is output to the writer. Variations of how the datais converted, and what values are assigned to the different pixels arelimitless and depend on what alterations to printed images is desired bythe user. Also, it can be seen that the rendering algorithm begins withor is based on detecting edges or edge pixels.

Referring to FIG. 4, a binary bitmap of six objects toned or printed inan array of pixels is illustrated. In each array, the center pixel isconsidered the PIQ. Presuming the PIQ is an edge pixel, there are sixdifferent relational configurations or objects defined as to where thePIQ is located with relation to the surrounding object. The sixpossibilities are provided as a directional look up table (DIR LUT) ordirectional LUT. Six variable values S, N, E, W, NE, SW, SE, NW areassigned the six configurations. It can be seen that in half the cases,the PIQ is a multi-edge pixel in that it is part of more than one edge.In the present embodiment, all other directional relationalconfigurations would be assigned a value of zero. The PIQ and adjacentobject may be identified using any of a number of edge detectionalgorithms, such as the Laplacian kernel described hereinbefore. Withthis analysis, determination of the orientation of the PIQ with respectto adjacent pixels can be made. Different pixel values may be assignedto the different orientations to achieve different print results.

The following represents Pseudo code for 1 pixel wide line pixel valueassignment decisions in accordance with the exemplary algorithm forblock 320 of FIG. 2:

If pixel from A is an edge pixel and pixel value from DIR LUT is 0,

Then pixel is part of 1 pixel wide line.

The following represents Pseudo code for a 2 pixel wide line pixel valueassignment decisions in accordance with exemplary algorithm for block320 of FIG. 2:

If pixel from A is an edge pixel, Then if pixel from DIR LUT is a E andif adjacent pixel to the right is a W     Then Pixel is part of a twopixel wide line   Else if pixel from DIR LUT is a SE and if pixel onnext line and to the right is a NW     Then Pixel is part of a two pixelwide line   Else if pixel from DIR LUT is a S and if pixel on next lineand directly below is a N     Then Pixel is part of a two pixel wideline   Else if pixel from DIR LUT is a SW and if pixel on next line andto left is a NE     Then Pixel is part of a two pixel wide line   Elseif pixel from DIR LUT is a W and if adjacent pixel to the left is a E    Then Pixel is part of a two pixel wide line   Else if pixel from DIRLUT is a NW and if pixel on previous line and to left is a SE     ThenPixel is part of a two pixel wide line   Else if pixel from DIR LUT is aN and if pixel on previous line and directly above is S     Then Pixelis part of a two pixel wide line   Else if pixel from DIR LUT is a NEand if pixel on previous line and to right is a SW     Then Pixel ispart of a two pixel wide line   Else pixel is an edge pixel

As described hereinbefore, the RIP provides image data to a rendercircuit 39. The RIP 37 and render circuit 39 can be dedicated hardware,or a software routine such as a printer driver, or some combination ofboth, for accomplishing this task.

The rendering circuit or algorithm of the present invention defines,classifies or identifies each pixel as a particular kind of pixel andreassigns pixel values as a function of their classification, where thedifferent classification reassignment values may be independent of eachother. For example, the algorithm may classify each pixel as either abackground pixel, interior pixel, edge pixel, one line pixel, or twoline pixel and reassign new values to these pixels according to thoseclassifications and independent of the each other. For example, interiorpixels may be reassigned new values while edge pixel remained unchanged,or edge pixels may be reassigned new values while leaving interiorpixels unchanged, or edge pixel values may be lowered with respect tointerior pixel values, or interior pixel values may be lowered withrespect to edge pixel values, etc. It can be seen there are unlimitedvariations to the present rendering algorithm. For example, therendering circuit or algorithm of the present invention may define eachpixel as either a background pixel, interior pixel or edge pixel andreassign these values independently of each other. Many pixelclassifications may be defined, examples of which have been definedherein with the designations background pixel (BP), foreground pixel(FP), interior pixel (IP), edge pixel (EP), one line pixel (1W), twoline pixel (2W), N, S, E, W, NE, NW, SE, SW, Y, Z, etc.

Referring to FIGS. 6 a–6 f, wherein a character is represented in apixel grid. FIG. 6 a is an illustration of a binary bitmap of acharacter. It can be seen that the pixels are either all black (filledwith solid area density of maximum toner d_(max)) or have no toner andhave area toner density of zero.

FIG. 6 b illustrates the toned character of 6 b after assigning a lowerpixel value to both interior pixels and edge pixels. In other words, IPVand EPV were reassigned from d_(max) in FIG. 6 a to d_(x), where d_(x)is lower than d_(max).

FIG. 6 c illustrates the edge pixels of the character when the characteris undergoing thinning.

FIG. 6 d illustrates assignment of new EPV and IPV values for the edgeand interior pixels of the character after thinning has occurred.

FIG. 6 e illustrates the edge pixels of the character when the characteris undergoing thickening.

FIG. 6 f illustrates assignment of new EPV and IPV values for the edgeand interior pixels of the character after thickening has occurred.

It can be seen from these figures that after operation of the algorithm,the edge pixels and interior pixels may be assigned grey levelsindependently. Once edge pixels are detected, the remaining pixelsconsist of either “background” pixels (white unprinted area) or“interior” pixels (foreground less edge pixels). Interior pixels can bedistinguished from background pixels in that if a pixel is NOT an edgepixel (from above) and if in the original image data the pixel is a 0(no marking) then the pixel is a background pixel. On the other hand, ifthe pixel is NOT a edge pixel (from above) and if in the original imagedata the pixel is a (marking) then the pixel is an interior pixel. Withthe rendering circuit, the exposure level of interior pixels can bechanged. Second and subsequent layers of edge pixels can be detected bysimply performing the edge detection algorithm on the interior pixelswhich remain after the edge pixels are removed. Interior pixels wouldthen refer to pixels remaining after all layers of edge pixels have beenremoved. A flow chart for this type of iteration is illustrated in FIG.17.

The steps taken in FIG. 17 begin with step 610 of receiving image bitmapdata. In a step 612, the edge pixels are identified and assigned a newvalue EPV₁ in a step 614 and thereafter sent to the writer. The Edgepixels identified in step 612 are also assigned a value of zero in astep 616, thereby creating a new “virtual” Edge 2. The Edge 2 pixels areidentified in a step 618 and reassigned a new pixel value EPV₂ in a step620 and thereafter sent to the writer. The Edge 2 pixels identified instep 618 are also assigned a value of zero in a step 622, therebycreating a new “virtual” Edge 3. The Edge 3 pixels are identified in astep 624 and reassigned a new pixel value EPV₃ in a step 626 andthereafter sent to the writer. This process can be iterated many timesover so that Edge N−1 pixels are assigned a value of zero in a step 628,thereby creating a new “virtual” Edge N. The Edge N pixels areidentified in a step 630 and reassigned a new pixel value EPV_(N) in astep 632 and thereafter sent to the writer.

A process similar to that described above process may be utilized tothicken or expand the size of an object edges by simply assigning avalue higher than zero, such as one or d_(max) in steps 616, 622, 628,etc. in order to create a new edge, real or virtual.

FIGS. 5 a–5 d illustrate different alterations that may be accomplishedusing an iterative edge detection, or edge “peeling” technique.

FIG. 6 b illustrates another technique for altering the image, which isto assign the same grey level to both edge and interior pixels.

Referring to FIGS. 7 a–7 d in conjunction with FIGS. 2 and 3, wherein asingle pixel width character or line is represented in a pixel grid.FIG. 7 a is an illustration of a binary bitmap of a one pixel widecharacter. It can be seen that the pixels are either all black (filledwith solid area density of maximum toner d_(max)) or have no toner andhave area toner density of zero. FIG. 7 b illustrates assignment ofeight bit values for the binary values of FIG. 7 a after determinationof the edge pixels according to a Laplacian kernel. FIG. 7 c illustratesassignment of direction values for the pixels surrounding characterpixels after application of the assignment algorithm described andillustrated hereinbefore in FIG. 4. FIG. 7 d illustrates the assignmentbackground pixel, edge pixel and of direction values for the pixel gridin accordance with the algorithm described and illustrated hereinbefore.

Referring to FIGS. 8 a–8 d in conjunction with FIGS. 2 and 3, wherein atwo pixel width character or line is represented in a pixel grid. FIG. 8a is an illustration of a binary bitmap of a two pixel wide character.It can be seen that the pixels are either all black (filled with solidarea density of maximum toner d_(max)) or have no toner and have areatoner density of zero. FIG. 8 b illustrates assignment of eight bitvalues for the binary values of FIG. 8 a after determination of the edgepixels according to a Laplacian kernel. FIG. 8 c illustrates assignmentof direction values for the pixels surrounding character pixels afterapplication of the assignment algorithm described and illustratedhereinbefore in FIG. 4. FIG. 8 d illustrates the assignment backgroundpixel, edge pixel and of direction values for the pixel grid inaccordance with the algorithm described and illustrated hereinbefore.

As described, in order to preserve fine lines (avoid loss ofinformation), one and two pixel wide lines are each detected when“thinning”. All pixels that comprise a one or two pixel wide line arecategorized as edge pixels after the laplacian operation. It is to beappreciated that many methods known in the art can be used to identify 1and 2 pixel wide lines. As described herein, to distinguish 1 and 2pixel wide lines from other edge pixels, the original image which hasbeen converted to 8 bits is operated upon by a 3×3 direction look uptable (DIR LUT). The resulting output contains information identifyingthe edge gradient of all edges. Using information from the originalimage, the output of this operation along with edge pixel data from theimage created by the laplacian operation is used to identify pixelswhich are part of a one pixel wide line from pixels which are part of atwo pixel wide line. Since one pixel wide lines can be detected anddistinguished from 2 pixel wide lines, each type of line can have aunique gray level assigned to it which in turn can be different fromother edge pixels.

Note that if onion skin layering is applied, there may exist first layeredge pixel values, second layer pixel values, etc. The gray level rangefor interior and edge pixels is 0 (no exposure) to 255 (maximumexposure). When thinning, one and two pixel wide lines have a range fromsome minimum exposure (not 0) to the maximum exposure. This is so thatthese lines will appear on the print. However, the present inventiondoes not preclude setting gray level on these in order to intentionallyerase fine lines.

FIG. 9 illustrates an example of an interface for an operator to adjustthe pixel density assignment values. Other inputs can be utilized. Asdiscussed previously, an operator can adjust these parameters indifferent ways to achieve a desired print result. For exemplary purposesonly, there is shown adjustments for the values of Interior Pixel, EdgePixel, One Pixel Wide, Two Pixel Wide, Toner Consumption, CharacterLinewidth, Shadow, Asymetry and Exposure Modulation(lightness/darkness). The adjustments can be made utilizing a userinterface or mechanical switch connected the printer, the particularkind and style of interface being variable. Providing a user with aninterface allows that user to make many adjustments to the image so asto achieve a particular print output without having to rerip the image.As discussed herein, different printers provide different printcharacteristics. The user interface provides a means to adjust oneprinter to mimic or appear like another printer on the fly, so to speak.That is, adjustments can be made while the printer is operating so thatprint output may be analyzed quickly and iteratively with littleinconvenience. Not all of the adjustments in FIG. 9 would be located inthe same interface, and other adjustments not specifically shown thereinare contemplated.

The present rendering circuit may be used in any type of electrographicsystem, of any size or capacity in which pixel exposure adjustment valueis selected prior to printing. The printer processes a bit map of theimage to be printed and identifies edge pixels first and then identifiesother types of pixels in that image. The exposure level for these pixelsis then set by the printer according to new pixel exposure adjustmentvalues according to density adjustments performed by the printer. Manyprinted image and object characteristics, parameters and utilities maybe affected by this method. For instance, a pattern may be provided tointerior pixels. This would be applied in the mapping section where theinterior pixel value is assigned. A benefit to the present algorithm isthat changes may take effect immediately because process controlcontrols to the same density.

When combining output from different printers to create one document, itis sometimes desirable to have the look and feel of the printers to beas similar as possible. Also, bitmaps of images ripped on one printerare sometimes printed on a printer with different characteristics thanthe original printer for which they were ripped. The present inventionprovides a method to obtain this result without reripping images andwithout adjusting other machine setup parameters (e.g.electrophotographic process setpoints). Appearance aspects which may beadjusted include but are not limited to text, line widths and pictorialtone scale. Feel aspect include but are not limited to toner stacking(tactile feel of toner stack). Image adjustments made utilizing therendering circuit described herein take immediate effect on print outputand therefore avoids any time delays normally associated with closedloop control system adjustment to electrophotographic process setpoints.

Sometimes users are willing to tradeoff image quality to attain highertoner yield per printed page. Another aspect of the rendering circuit isto provide the user with a “knob” or adjustment to adjust tonerconsumption at various levels of image quality, as shown in FIG. 9. Auser is provided the ability to lower certain pixel values, likeinterior and edge pixels, thereby lowering the amount of toner beingdeposited in the affected pixels and thereby lowering overall tonerconsumption. A user can adjust the printed image in this manner so as tominimize toner consumption while maintaining acceptable image qualitywithout having to rerip the image.

To this end, it can be seen that the rendering circuit accounts for allpixels of an image to be printed, and determines toner levels for eachpixel. With this being the case, the printer may track or monitor totaltoner consumption of the printer accurately by adding or calculating thetoner deposited for each line, character, and image processed andprinted. By counting the number of edge (those having at least oneadjacent pixel non toned) and interior (those having all adjacent pixelstoned) and applying different conversion factors (toner usage per pixelto each), a prediction of toner usage can be achieved. Toner consumptionby line, page, job or multiple jobs can be accomplished. This estimatehas customer applications as well as potential uses in tonerreplenishment/toner concentration control in the printer itself. To thisend, toner consumption can be estimated/calculated in real time and theinformation can be used to replenish toner more accurately to therebyreduce density variations which improves image quality. The conversionfactors applied can also be dependent on the density targets used inprinters that have variable density control allowing the customer toselect the best cost/quality point for each job. As an example, 6%coverage documents made up of text and made up of inch solid squareshave been shown to consume between 0.0397 and 0.0294 grams of toner persheet respectively. This difference of 33% occurs even though the totalnumber of black pixels for the two documents differs by less than 0.5%Analyzing these two images for edge and interior pixels indicates thatedge pixels consume 1.3 times the toner that interior pixels do.Accounting for the edge and interior pixels separately clearly yieldsimproved estimates for toner consumption than estimates using only pixelcounts.

As mentioned hereinbefore, the process of electrography involves formingan electrostatic charge image on a dielectric surface, typically thesurface of a photoconductive recording element that is being drawn orotherwise conveyed through a developing station or toning zone. Theimage is developed by bringing a two-component developer into contactwith the electrostatic image and/or the dielectric surface upon whichthe image is disposed. The developer includes a mixture of pigmentedresinous particles generally referred to as toner andmagnetically-attractable particles generally referred to as carrier. Thenonmagnetic toner particles impinge upon the carrier particles andthereby acquire a triboelectric charge that is opposite the charge ofthe electrostatic image. The developer and the electrostatic image arebrought into contact with each other in the toning zone, wherein thetoner particles are stripped from the carrier particles and attracted tothe image by the relatively strong electrostatic force thereof. Thus,the toner particles are deposited on the image. The magnetic carrierparticles are drawn to the toning shell by the rotating magnets therein.This magnetic force generally does not affect the nonmagnetic tonerparticles.

However, within the toning zone the toner particles are affected byforces other than the electrostatic force attracting the toner to theimage and which may degrade image quality. These forces include, forexample, repulsion of toner from the portion of the dielectric surfaceor photoconductive element that corresponds to the background area ofthe image, electrical attraction of the toner particles to the carrierparticles, repulsion of toner particles from other toner particles, andelectrical attraction to or repulsion from the toning shell depending onthe polarity of the film voltage in the developer nip area. There arecertain methods of compensating for and/or balancing the effect of theseother forces on the nonmagnetic toner particles to prevent anysignificant adverse effect on image quality. However, the forces ontoner particles having magnetic content are very different from theforces on nonmagnetic toner.

In addition to the electrical forces acting on nonmagnetic toner asdescribed above, toner having magnetic content is subjected to magneticforces, such as, for example, magnetic attraction of the toner particlesto the carrier particles, to other toner particles, and to the rotatingcore magnet. All of these magnetic forces are generally in a directionaway from the film or electrostatic image carrier. The only force actingto draw the toner onto the electrostatic image carried by the film ordielectric carrier is the electric force. Thus, the magnetic forces tendto counteract the electric attraction of toner particles to the image.The strength of the electric force relative to the magnetic forcesbecomes stronger as the distance between the image and the core magnetincreases. Therefore, the toner tends to be deposited on the trailingedge of the film or dielectric carrier. The result is an image havingsolids with heavy toning on the trailing edge of the image, and crosstrack lines (i.e., lines perpendicular to the direction of travel of thedielectric support member or film) that are wider than the correspondingin track lines (i.e., lines that are parallel to the direction of travelof the dielectric support member or film).

This “Fringe” field effect (the condition wherein fringe electromagneticfields around the edges of lines on the photoconductor result in tonerbuild up at edges of lines on the printed material) can be a problem forsome printers. The rendering circuit described herein provides a methodto reduce the toner build up on the edges by adjusting the IPV, EPV, 1PVor 2PV parameters accordingly to reduce or counteract these effects. Forexample, FIGS. 5 a–5 d illustrate a character having different exposurevalues assigned to different layers which may be utilized to minimizethe fringe field effect on image quality.

As described hereinbefore, d_(max) control uses the signal from atransmission densitometer circuit reading a d_(max) patch to adjust V₀and/or E₀ electrophotographic parameters concurrently to maintain solidarea density. In addition to d_(max), a shadow detail patch may bewritten using approximately 70–90% pixel pattern or at 70–90% of d_(max)exposure in a flat field pattern at the selected edge and interior pixelexposure values determined by the rendering circuit during tuning priorto the run. Based on the densitometer signal generated by this patch,the edge and interior pixel exposure values may be adjusted to maintainthe desired shadow detail density (or large line character width) byadjusting or reassigning pixel values. In addition, a highlight detailpatch may be written using approximately 5–20% of d_(max) exposure blackpixels in a flat field pattern using the selected edge, interior, andsmall feature pixel exposure values determined by the rendering circuitduring tuning prior to the run. Based on the densitometer signalgenerated by this patch, the small feature pixel values may be adjustedto maintain the desired highlight detail density (or fine line characterlinewidth) by reassigning one or more of EPV, 1PV and 2PV.

As described herein, it is possible using the rendering circuit to applyreduced exposure at all edges of characters, but this may lead to toolarge a reduction in line width since the minimum adjustment is appliedto two pixels. This is especially true of characters printed in a smallfont size. To achieve less linewidth reduction, half of character edgesmay be reduced (top and left edges only for example). This may lead,however, to an apparent shift of the center of the characters locationsand this may be undesirable for a particular application (for instancewith kerned fonts and small font size characters). To achieve linewidthreductions less than those achieved with all edge pixel exposurereductions, and avoid apparent center shifts of small font sizecharacters caused by top/left or bottom/right edge exposure reductions,the rendering circuit may apply an alternative algorithm and assignpixel values such that closed characters (those having enclosed spacessuch as o, d, b, etc.) have reduced exposure only for the interior orexterior edges of enclosed areas. For example, FIG. 10 illustrates aletter “O”, (which is a closed character), having interior edges andexterior edges with different exposure values assigned to them. Thishelps to maintain the center location of character without achievingexcessive linewidth reduction. Remaining straight portions of thecharacters may have only one edge exposure reduced. A similar algorithmmay be applied to characters having partially enclosed spaces (such asv, c, m, n etc.) whereby only the interior or exterior edge is exposuremodified. Characters with multiple partially enclosed spaces (such as t,y, w, m, etc.) would require a larger set of rules to avoid modifyingboth edges of any strokes, but it should be possible to generate aconsistent set of rules capable of avoiding such conflicts.

Desired edge exposure reductions may utilize a two dimensional operatorof sufficient size to completely enclose the largest size character towhich it will be applied. If an area is identified in the operator fieldof view as a separate object, it may then operate on the object inaccordance with the rendering algorithm described herein to reduceapparent linewidth while minimizing the apparent center shifting ofcharacters.

As the interior pixel (solid area density) exposures drop below certainlevels, electrophotographic process nonuniformities become apparent inthe solid area imaging. Assigning a pattern of different exposure valuesfor interior pixels (multiple IPVs rather than using a single exposurefor all interior pixels) reduces the visibility of EP processnon-uniformity. The particular pattern used is analogous to a halftoningpattern for binary imaging, except the modulation is between differentnon-white exposure levels. The pattern of differing density pixels tendsto obscure streaks and bands that become visible in flat fields of samelevel exposure pixels and minimizes the visibility of non-uniformdensity. The nonuniformities can be identified or measured in a numberof ways, examples of which are visually inspecting the printed output orutilizing a density patch and measuring density thereof. The pattern canbe of any size with any number of different exposure values such that itcreates the desired average interior pixel density when printed toreduce print nonuniformities.

In this regard, the present invention is useful when printing magnetictoner or ink. Magnetic Ink Character Recognition (MICR) technologieshave been used for many years for the automated reading and sorting ofchecks and negotiable payment instruments, as well as for otherdocuments in need of high speed reading and sorting. As well known inthe art, MICR documents are printed with characters in a special font(e.g., the E13-B MICR font in the United States, and the CMC-7 MICRstandard in some other countries). Typically, MICR characters are usedto indicate the payor financial institution, payor account number, andinstrument number, on the payment instrument. In addition to the specialfont, MICR characters are printed with special inks or toners thatinclude magnetizable substances, such as iron oxide, that are magnetizedfor facilitating an automatic reading process by a reading instrumentwhich is sensitive to the magnetic fields surrounding the printed MICRcharacters. The magnetized MICR characters present a magnetic signal ofadequate readable strength to the reading and sorting equipment, tofacilitate automated routing and clearing functions in the presentationand payment of these instruments.

The relatively heavy loading of iron oxide in conventional MICR tonerfor electrographic MICR printing has been observed to adversely affectthe image quality of the printed characters, however. It is difficult toachieve and maintain an adequate dispersion of the heavy iron oxideparticles in the toner resin. In addition, the toning and fusingefficiencies of MICR toners are poorer than normal (i.e., non-MICR)toners, because of the magnetic loadings present in the MICR toner.Accordingly, the image quality provided by MICR toner may be poorer thanthose formed by normal toner, unless the printing machine makesadjustments to compensate. The present rendering circuit provides a wayto adjust MICR toner density in parts of characters so as to minimizethe printing nonuniformities resultant therefrom. By varying pixel tonerdensity values as a function of pixel character location as illustratedin the exemplary drawings herein, the concentration of magnetic tonerparticles may be adjusted to improve the readability of the printedcharacters by reading instrumentation.

FIG. 11 illustrates an example of a typical tone reproduction curve,also referred to in the art as a “gamma” curve, illustrating the typicalperformance of conventional printers in reproducing tone density, inthis example a gamma curve for gray scale printing. In this plot, thehorizontal axis corresponds to input intensity between white (nointensity) and black (full intensity); the vertical axis corresponds tothe corresponding printer output density, on the hard copy medium,between d₀ (no density) and d_(max) (full density). Ideally, thetransfer function from input intensity to output density would be a 45°line, shown as ideal plot I in FIG. 11, along which the output densityexactly matches the input intensity.

Printer performance follows a non-linear “S-shaped” tone reproductioncurve, for example as shown by actual plot A in FIG. 11, often referredto as the “gamma” curve. Along this tone reproduction curve, outputdensity is generally less than that specified by low input intensityvalues (i.e., below the ideal I); this portion of the tone reproductioncurve is referred to as the “toe” shown by region T in FIG. 11. Theoutput densities in the “toe” region are also referred to as “highlight”densities. At the other extreme, for high input intensity values, outputdensity is generally higher than that specified by the input (i.e.,above the ideal I). These output densities in the “shoulder” region ofthe tone reproduction curve, for example in region S of plot A in FIG.11, are also referred as “shadow” densities. For both the highlight andshadow densities, the inaccuracy in tone reproduction is generallymanifest by inaccuracies in the printed contrast; the underdensity inhighlight regions shows up as washed out regions of the image, while theoverdensity in shadow regions shows up by the absence of bright features(loss of detail in dark regions). In the “midtone” region of the tonereproduction curve, shown by region MT of plot A in FIG. 11, the errorbetween output density and input intensity is relatively small, so thatmidtones produced by the printer closely match the input signal.

In many cases, the Raster Image Processor (RIP) described above, by wayof which a page description is converted into a bit map output forprinting by a specific printer of the electrographic or other type,applies gamma correction in this processing. This gamma correctioncompensates for the non-ideal density output of the printer, in effectapplying a transfer function that is the opposite of the tonereproduction curve for the printer (e.g., plot A of FIG. 11). Thiscorrection will generally be implemented by increasing the densityoutput for lower input intensity values, and decreasing the densityoutput for higher input intensity values. To at least a firstapproximation, the correction amounts to the selection of a gamma value,which is a compensating factor corresponding to the degree of curvatureof the actual tone reproduction curve A from the ideal I. As notedabove, the actual correction may be carried out by selection of theappropriate halftone screens using higher density halftone screens forhighlight densities, and lower density halftone screens for shoulderdensities.

According to conventional approaches, the selection of the appropriatehalftone screens for a given printer or printer type requires a trialand error process. The correct d_(max) output density level must firstbe correlated to full density input. Once d_(max) is set, then arepresentative image is processed using a trial set of corrections forhighlight and shadow densities; after analysis of the output image, thecorrections may be adjusted and the image processed again. Uponconvergence to the desired output, additional images may be adjustedusing the corrections (e.g., the selected set of halftone screens)determined in the trial and error process, and printing can commence. Tothe extent that the iterative setting of shoulder and toe correctionsmust be performed for a given printer, or on specific images, thisprocedure is time consuming and costly.

Because of printer specificity in the RIP process, RIP output for oneprinter or printer type cannot be forwarded to a differentelectrographic printer without risking that the printed image will haveincorrect gamma correction for the images. In other words, the gammacorrection in the RIP output based on the printer for which the originalRIP was performed will likely not correspond to the tone reproductioncurve of a different printer.

As discussed above, U.S. Pat. No. 6,121,986 provides a solid areadensity control system, in which the optical density of maximum densitypatches, and of less than maximum density patches, is controlled inresponse to the measured performance of the electrographic printer. Thissolid area density control adjusts the output density d_(max) duringsetup and operation of the printer, and also can control the outputdensity at different less-than-maximum levels. However, thisconventional solid area density control only controls the solid areaoutput density value d_(max), and cannot separately control highlightand shadow densities. In other words, an increase in solid area outputdensity d_(max) compensates for the underdensity of highlights, butovercompensates for shadows. Conversely, a decrease in d_(max)compensates for the overdensity of shadows, but undercompensates forhighlights. While solid area control approaches stabilize the opticaldensity of the exposed areas, they don't necessarily introducevariations into character linewidths of text (and analogously into thelinewidths of small isolated image features). Linewidth variations aredue in part to fringe field effects. As known in the art, the amount oftoner applied to a pixel on the photoconductor of an electrographicprinter depends upon the difference between the exposure voltage (asapplied by the LED or laser to the photoconductor) and the bias voltageat the toning station; changes in either of these voltages will changethe amount of toner received by the pixel. Fringe effects occur becausethe electric field at the edge of an exposed patch (i.e., those edges ofexposed pixels that are adjacent to unexposed pixels) is much greaterthan the field at the center of the exposed region. It has been observedthat the difference in field magnitude between the edge and the centermay be as high as 3× to 5×. As a result, toner tends to pile up at theedge of an exposed patch of pixels, and at the edge of single exposedpixels surrounded by unexposed pixels. In the case of single pixels,this piling effect can result in single pixel sizes of on the order of90μ in 600 dpi printers that have a theoretical pixel pitch of 42μ.Again, these fringe effects affect both gray scale images and alsofull-black text and make it difficult to adjust image quality to theextent necessary to compensate for differences in characteristicsbetween an electrographic printer for which the image was originallyRIPped, and a different electrographic printer upon which the image isto be printed. These fringe effects are reduced utilizing the renderingcircuit of the present invention by reassigning edge pixels to havelower exposure values (EPV) at the edge of an exposed patch of pixels,and at the edge of single exposed pixels surrounded by unexposed pixels.

Digitized halftone images processed at different effective screenfrequencies (the number of lines per inch or lpi) often have differentcontrast (appearances) because of differing dot gains depending on theratio of edge and interior pixels as the area coverage changes. FIG. 12illustrates seventeen halftone steps (the percentage of white in eachstep) for three different screen frequencies, 106 lpi, 85 lpi and 71lpi. The relationship of percent lightness to percent black pixels foreach step for each screen frequency is shown in FIG. 13. It can be seenthat the three curves at standard exposure are different, therebyillustrating different halftone images for different screen frequencies.FIG. 14 illustrates a series of lines that are 1, 2, 3, 4, and 8 pixelswide, respectively. FIG. 15 illustrates a graph of linewidth vs thenumber of pixels counted across the line, where white spaces areassigned negative numbers for a particular set of lines with the samelinewidth (for example 8 pixel wide lines). A best fit line 500 can bedrawn through the data points collected. FIG. 16 illustrates a series ofbest fit lines extracted for linewidth vs the number of pixels derivedby selecting a fixed IPV and varying EPV, 2PV and 1PV for eightdifferent cases. It can be seen that there are eight different best fitlines. It can also be seen that there is one particular best fit linethat passes through the zero intercept. The EPV, 2PV and 1PV values forthe zero intercept line was noted and a series of lines similar to thoseshown in FIG. 14 were printed at screen frequencies of 106 lpi, 85 lpiand 71 lpi. The relationship of percent lightness to percent blackpixels for the three screen frequencies were plotted and are shown inFIG. 13, wherein the resulting curves identified as the zero interceptgroup curves. It can be seen that using the EPV, 2PV and 1PV values forthe zero intercept line results in digitized halftone images that arethe same for differing screen frequencies. By using EPV, 2PV and 1PVexposures that are different from IPV exposure, it is possible toachieve linear behavior between character linewidth and the number ofpixels printed that has an intercept of zero. Because the IPV exposurehasn't changed, it is possible to retain good solid area fill byoverlapping interior pixels. Because the relationship between pixelwidth and measured width has a zero intercept, image density forhalftone patterns is not dependent on the ratio of edge and interiorpixels, which means that is it also independent of screen frequency.Using a user interface, the user is therefore able to adjust the solidarea maximum density (IPV) and then select edge pixel exposures (EPV,2PV, 1PV) to achieve a zero intercept of the character linewidth vnumber of pixels curve to minimize screen frequency sensitivity. To thisend, sensitivity to screens having different dot shapes (e.g. round,elliptical, diamond, etc.) may be minimized also.

While the present invention has been described according to itspreferred embodiments, it is of course contemplated that modificationsof, and alternatives to, these embodiments, such modifications andalternatives obtaining the advantages and benefits of this invention,will be apparent to those of ordinary skill in the art having referenceto this specification and its drawings. It is contemplated that suchmodifications and alternatives are within the scope of this invention assubsequently claimed herein.

1. A method of estimating toner consumption of a digital image whenprinted, the digital image comprising a plurality of pixels, and themethod comprising the steps of: determining a pixel type from aplurality of pixel types for each of the plurality of the pixels,wherein the plurality of pixel types includes a background pixel, aninterior pixel, a first type of edge pixel, and a second type of edgepixel; assigning a toner consumption value to each of the plurality ofpixels based at least upon each pixel's determined pixel type; adding atleast the toner consumption values of each of the plurality of pixels toarrive at a sum; and estimating toner usage based at least upon the sum.2. A method in accordance with claim 1, wherein the digital image is abinary image.
 3. A method in accordance with claim 1, wherein thedigital image is a multi-bit image.
 4. A method in accordance with claim1, wherein the first type of edge pixel is a one-pixel-width line, andthe second type of edge pixel is a two-pixel-width line.
 5. A method inaccordance with claim 1, wherein the plurality of pixel types furtherinclude a third type of edge pixel.
 6. A method in accordance with claim5, wherein the first type of edge pixel is a one-pixel-width line, thesecond type of edge pixel is a two-pixel-width line, and the third typeof edge pixel is an edge pixel other than a one-pixel-width line andother than a two-pixel-width line.
 7. An apparatus for printing animage, the apparatus comprising: a raster image processor for convertingthe image into a digital bitmap comprised of a plurality of pixels; arendering circuit for receiving the digital bitmap from the raster imageprocessor for; determining a pixel type from a plurality of pixel typesfor each of the plurality of the pixels, wherein the plurality of pixeltypes includes a background pixel, an interior pixel, a first type ofedge pixel, and a second type of edge pixel; assigning a tonerconsumption value to each of the plurality of pixels based at least uponeach pixel's determined pixel type; adding at least the tonerconsumption values of each of the plurality of pixels to arrive at asum; and estimating toner usage based at least upon the sum.
 8. Theapparatus of claim 7, further comprising a writer interface whichreceives the toner consumption values and facilitates exposure of alatent image on an exposure medium.
 9. The apparatus of claim 7, whereinthe first type of edge pixel is a one-pixel-width line, and the secondtype of edge pixel is a two-pixel-width line.
 10. The apparatus of claim7, wherein the plurality of pixel types further include a third type ofedge pixel.
 11. The apparatus of claim 10, wherein the first type ofedge pixel is a one-pixel-width line, the second type of edge pixel is atwo-pixel-width line, and the third type of edge pixel is an edge pixelother than a one-pixel-width line and other than a two-pixel-width line.